This invention is concerned with novel coating compositions, with processes for their application and use, and with articles which are formed thereby. More particularly, the invention is drawn to coatings which are useful in the art of photography and photolithography and especially to a new genus of coatings for photo-resistive, photographic and other purposes which employ diacetylenes. According to a preferred embodiment, such coatings may be formulated which comprise a plurality of domains, which domains exhibit a uniquely regular structure; such embodiment yields photographic films and photolithographic materials of exceedingly high resolution and efficiency.
The diacetylenic compositions taught hereby offer vastly improved properties for photography and photolithography as compared with the materials known to the prior art. Thus, greatly improved efficiency, impermeability, structural regularity, resolution, contrast, and adhesion to substrates are evidenced. A direct comparison with commonly employed photoresistive materials may be had. While known materials such as glycidyl methacrylate, polybutene sulfone and polymethyl methacrylate have quantum efficiencies on the order of 0.1 or less, the diacetylenic species taught by the present invention have quantum efficiency from 10.sup.8 to 10.sup.12 molecules reacted per photon. Similarly, while the prior art materials require a radiation absorption of from 10-20 to 300-500 joules per cubic centimeter to achieve the cited quantum efficiencies, the diacetylenes of the present invention require only about 0.03 joules per cubic centimeter. Additionally, while currently used photo-resists have resolving powers of about 10,000 .ANG., and 2000-3000 .ANG. in extreme cases, the resists of the present invention may resolve structure as small as 100 .ANG.. Thus, it will be readily appreciated by those skilled in the art that the diacetylenic coating compositions taught by the present invention represent a very substantial advance in the photographic and photolithographic arts.
Photo-lithography finds its roots in 1852 when British Pat. No. 565 was issued to Talbot for a bichromate sensitized gelatin coating which was useful for differential etching of copper by ferric chloride. Since this time, numerous compositions and processes have been developed which serve to transfer a pattern of light or other radiant energy onto a substrate through the intermediation of a coating sensitive to such radiant energy. The art of photolithography has developed broadly in recent decades to include various modes of printing, etching, and image reproduction from macro to micro scales, and has fostered the development of entire new industries. See generally, Photo-Resist Materials and Processes by William DeForest, McGraw-Hill 1975.
One industry which has placed heavy reliance upon the use of photolithography is that of microstructure fabrication. This industry which produces structures imposed upon surfaces of thin films having extremely small dimensions is the basis for the production of information processing, electronics, switching, and other devices which rely upon "integrated" electronics, the fabrication and interconnection of large numbers of very small device structures on a single piece of silicon or other substrate.
Exemplary uses of photolithography employing what is commonly denominated in the art as photo-resists, are described in Science, volume 196, No. 4293, pp 945-949, May 27, 1977, by R. W. Keyes. As taught in the Keyes article, a substrate, usually silicon, is caused to be coated with a layer of electrically non-conductive material, frequently silicon dioxide, usually by "growth" from the substrate itself through a suitable oxidizing process. The substrate is then further coated with one or more layers of photo-resist. At this time, selected portions of the photo-resist are exposed to a suitable form of radiant energy, which irradiation causes an alteration of the structure or physical characteristics of the portions of the resist thus exposed. Thus, electromagnetic energy such as light, especially ultraviolet light; X-radiation, laser radiation, etc. and particle radiation such as electron beam, particle beam, and plasma radiation may be utilized to alter the structure or physical characteristics of the portion of the photo-resist irradiated.
Following the irradiation of the selected portions of the photo-resist and concomitant alteration of the structure or physical characteristics thereof, a distinction may be drawn between the selected portions of the resist and those not selected based upon the altered structure or physical characteristics. Thus, the selected portions may be viewed as being either more or less suited to removal from the underlying substrate than are the unselected portions through any appropriate means. The selected portions may be either more or less soluble in a liquid medium, may have a higher or lower vapor pressure, may exhibit greater or lesser resistance to chemical attack, or may evidence any other differing structure or characteristic which may facilitate differential removal from the substrate as compared to the unselected portions. Based upon one or more of these distinctions, the selected (irradiated) portions of the photo-resist are either removed from the substrate while the unselected (unexposed) portions remain, or the selected portions are retained while the unselected portions are removed. By analogy with traditional photographic processes, the former process is carried out with photo-resists denominated as "positive" resists while the latter employ "negative" photo-resists.
The selection of certain portions of a photo-resist for irradiation may be accomplished in various ways, all of which are well known to those skilled in the art. Masking is a common procedure which is performed by placing over the surface of a photo-resist-coated substrate (or composite substrate) a mask, usually formed photographically, and by causing the passage of radiation through the mask onto the resist layer. The mask is designed so as to have transmissive and non-transmissive areas corresponding to the portions of the resist layer which have been chosen for irradiation and for non-irradiation respectively. As is well known to those skilled in the art, the radiant energy used in conjunction with the masking technique may benefit from being collimated, focused, monochromatized, or rendered coherent. Such persons will also recognize that all portions of the electromagnetic spectrum may be employed with the masking technique as may various forms of particle irradiation such as electron beams, ion beams, and plasma irradiation.
It is also well known to irradiate selected portions of a resist layer without the use of masking. Thus direct projection of a radiant energy beam, especially a laser, electron, or particle beam may be undertaken to irradiate the selected portions of the layer. Such projection is usually controlled by automated means and often by a computer. It is to be understood that the means and mechanisms for irradiating selected portions of the photoresist layer are well known in the art. See, for example, proceedings of the IEEE, volume 62, No. 10, pp 1361-1387, October, 1974, Henry I. Smith, Fabrication Techniques for Surface-Acoustic-Wave and Thin-Film Optical Devices; X-Ray Optics: Applications to Solids (Ed. H. J. Queisser); E. Spiller and R. Feder, X-ray Lithography, pp 35-92 Springer (1977); and Ann. Rev. Mat. Sci., vol. 6, pp 267-301, L. F. Thompson and R. E. Kerwin, Polymer Resist Systems for Photo-and Electron Lithography.
It may be seen that following the irradiation of the selected portions of the photo-resist and the selected removal either of the selected portions or of those portions not selected, a pattern will remain on the substrate formed of irradiated or non-irradiated photo-resist depending upon whether a negative or positive resist formulation was employed. This photo-resist pattern will alternate with exposed substrate, usually silicon oxide. The exposed silicon oxide (or other) substrate) layer segments or portions are then subject to treatment. In nearly all cases, the oxide layer is removed or etched away in manners well known to those skilled in the art. The etching away of the silicon oxide layer is designed to expose the underlying silicon substrate to treatment and processing. The etching of the silicon oxide layer is intended to be accomplished only in the areas not covered with portions of photo-resist. The photo-resist must therefore be relatively insensitive to the etching means employed to remove the exposed oxide portions.
After the etching off of the oxide layer the remaining photo-resist is removed to yield a pattern of exposed silicon and silicon oxide or other substrate formulation. The exposed silicon is then treated to alter its electrical characteristics, frequently through processes denominated as "doping". "Doping", which is well known to those skilled in the art, causes elements such as phosphorous or boron to be diffused into the exposed silicon or other substrate material. The treatment of the article after the removal of the photo-resist is not directly related to this embodiment of the invention and is well known to those skilled in the art. The treated article may subsequently be re-coated with photo-resistive materials, exposed selectively, and caused to undergo differential removal of resist, etching, and treatment one or more additional times to result in complex surface structures on the substrate. A selectively exposed and removed resist layer may also be used for the deposition of metal or other conductive materials onto the surface of the substrate to serve as electrical contacts with the variously treated surface areas as is well known to those skilled in the art.
It is to be understood that the foregoing background is not intended as a rigorous delineation of the metes and bounds of the microfabrication art and that numerous variations, both of materials and processes are known. It should be further understood that the term "photo-resist" has been adopted by those skilled in the art as a generic term descriptive of all forms of radiation--sensitive coatings useful in lithography. Thus, the term embraces coatings useful for microfabrication, etching, printing, and compositing as well as numerous others. Similarly, the term "photo-resist" has been adopted for those coatings which are sensitive to particulate radiation such as electron beams.
It is apparent from the foregoing that the characteristics of the photo-resists useful in a particular photolithographic process is of central importance to the success of that process. In general, it is recognized by those skilled in the art that the ideal photo-resist will exhibit certain exemplary qualities. Thus, a photo-resist should be capable of ultra-fine resolution, and should exhibit a high response to incident radiation. Additionally, an exemplary photo-resist should be easily applied to substrates, should coat such substrates uniformly and should adhere to them firmly until removal is desired. Further requirements are that the resist, when exposed to incident radiation, should undergo a substantial change in physical properties, thus to provide a clear distinction between the exposed and unexposed portions so as to facilitate the selective removal of one or the other from the substrate when desired.
As has been indicated, numerous compositions for use in photo-resistive processes have been developed. As has also been explained, these fall generally into the "positive" or "negative" resist categories depending upon whether selective removal or selective retention of the exposed portions of resist is contemplated. Negative photo-resists, in general, operate through a bond formation mechanism, usually a crosslinking or polymerization. Thus such resist formulations generally comprise monomers which polymerize or polymers which crosslink upon exposure to radiation. It may thus be seen that the effective molecular weight of the system is increased in negative resist formulations such that differential removal of those portions not irradiated is facilitated. A frequent means for effecting such removal is solvent dissolution, but other means such as vaporization may also be employed. The positive resists on the other hand operate usually via a bond cleavage mechanism whereby the molecular weight is decreased. Thus the exposed portions of positive resist formulations are differentially removed in such systems.
The materials and processes of this invention are also admirably suited for employment in the photographic art. Those skilled in the art will understand that the beneficial properties of the materials and the advantageous aspects of the properties are also applicable to photographic systems. Thus, photographic materials and systems having excellent sensitivity, resolution and efficiency may be formulated with these materials and processes. In addition, the materials of the invention are known to display a wide spectrum of vivid colors upon polymerization thus leading to novel color photographic materials.
It will be seen that preferred embodiments of the materials and processes of this invention employ monolayer and multilayer structures which comprise pluralities of domains of the materials, which domain have regular structures. Such domain assemblages are thought to lead, in part, to the superior properties displayed herein.